1. Field of the Invention
This invention relates to a stereoscopic microscope for observing therethrough an object from an oblique direction.
2. Description of the Prior Art
When an object is observed at a comparatively low magnification for a long time, use is usually made of a stereoscopic microscope to reduce the viewer's fatigue and to grasp the stereoscopic construction of the object. The observation is usually effected from the direction perpendicular to the plane of the object, but in some cases it is required that the observation be effected from a direction oblique with respect to the plane of the object for some reason or purpose. For example, such cases include the case of a microscope used to examine wafers during the manufacturing process of Integrated Circuits (hereinafter abbreviated as IC). In this case, various patterns are successively printed on IC wafers and such printing is effected vertically from above the wafer and so, the wafers must be observed from an oblique upward direction so as not to interfere with the printing.
As a microscope for effecting such stereoscopic observation from the oblique upward direction, there is known an application of the so-called inwardly oblique stereoscopic microscope as shown in the perspective view of FIG. 1 of the accompanying drawings, but satisfactory observation has been impossible because of distortion of the image, as will hereinafter be described. In such case, the observation optic axis of the eyes (the direction in which the object plane is observed, and in the present case they are coincident with the optic axes of objective lenses L10 and L'10, respectively) is converged at an angle of 2.alpha., and for the observation from the oblique direction, the plane S containing the optic axes is inclined by an angle .beta. with respect to the normal n to the object plane.
These angles of inclination .alpha. and .beta. are within planes orthogonal to each other, so that distortion resulting from the difference in magnification takes place in the two directions orthogonal to each other, and when an object like graph paper as shown in FIG. 2(a) is to be observed, the right and left observation images become configured as roughly shown in FIG. 2(b). The distortion in the y-direction of the two right and left images is the distortion in the same direction resulting from the inclination .beta. of the observation optic axis, while the distortion in the x-direction is oppositely directed distortion resulting from the angle of conversion 2.alpha. and thus, no parallel straight line exists in the two images. Therefore, if the right and left images are superposed upon each other with a certain straight line in the x-direction as the reference, the straight line of the right and left images in the y-direction intersect each other and if certain straight lines in the y-direction are made coincident with one another, the straight lines in the x-direction intersect each other, thus making it impossible to make coincident any two straight lines in the right and left images. Therefore, if stereoscopic observation is effected, the fields of view in the two eyes are not coincident at all and this has caused great fatigue of the viewer's eyes and consequent headache, thus bringing about great inconvenience in long-time observation.
On the other hand, relatively high magnifications are also required in a stereoscopic microscope for observing an object from an oblique direction and in such case, the field of view necessarily becomes narrow, thus making it impossible to observe a wide range. Moving the object would occur to mind to enable a wide range to be observed and for example, in the observation of IC wafers described above, moving the wafer itself during the printing is nearly impossible because the printing of various patterns on the wafer is effected while very precise positioning is taking place. It is therefore necessary to contrive continuously to vary the field of view in the microscope.
For this purpose, it would occur to mind to vary the inclination of the observation optic axes about the objective lens, but the conventional device as shown in FIG. 1 is not sufficient for such variation. FIG. 3 shows, in side view, the manner in which image formation takes place when the angle of inclination of the observation optic axes in the conventional device is .beta..sub.1. The dash lines in FIG. 3 refer to the case of the angle of inclination .beta..sub.2. Since the right and left observation optical systems are equivalent to each other, description of only one of them is sufficient for the following explanation. When the angle of inclination of the observation optic axis changes from .beta..sub.1 to .beta..sub.2, the center of the view field should change from the object point P.sub.1 to P.sub.2, as shown. However, the distance to the object point P.sub.1 and the distance to the object point P.sub.2 change with respect to the objective lens L10, so that the distance to the image point P'1 and the distance to the image point P'2 change, and also the inclination of the image plane Y is varied. Due to the Scheimpflug condition, known as the so-called swings and tilts, the image plane Y is formed so as to intersect the line of intersection (shown as point of intersection Q) between the object plane X and the principal plane H of the objective lens and therefore, the inclination of the image plane Y is varied more than the inclination of the observation optic axis. When the field view is moved by so varying the inclination of the observation optic axis, the image changes both its inclination and position with respect to the eyepiece as long as the objective lens and the eyepiece are integral, so that observation is only permitted within a very narrow range in the depth of focus of the objective lens L10 or the eyepiece L20, and sometimes no observation can be effected. By varying the inclination of the objective lens and moving the eyepiece so as to trace the resultant displacement of the image, it is not impossible to obtain a clear image but the movement of the objective lens and that of the eyepiece are entirely different from each other and this would necessitate a greatly complicated mechanism which would not be practical.